1. Field of the Invention
The present invention relates to an audio decoding device for decoding stream data containing encoded audio data.
2. Description of the Related Art
Some digital television broadcasting services (e.g., in Europe or the like) use stream data containing encrypted main audio (stereo), and in addition, stream data containing encrypted auxiliary audio used for explanation of a broadcast content.
A device for receiving the digital television broadcast combines main audio data (stereo) and auxiliary audio data obtained by decoding these stream data before outputting them. In this case, if the main audio data and the auxiliary audio data are simply combined, the loudness becomes larger than necessary, which is not comfortable for listening. Therefore, it is necessary to reduce the loudness of the main audio data to some extent before combination so as to obtain an appropriate loudness after combination. Also, if the sound localization of the auxiliary audio data can be controlled, it is convenient for listeners.
Therefore, the stream data of the auxiliary audio data contains parameters for processing the main audio data and the auxiliary audio data to adjust the relationship in loudness between the main audio data and the auxiliary audio data, and controlling the sound localization of auxiliary audio. An audio decoding device for decoding these stream data uses the parameters to adjust the loudness and control the sound localization (e.g., Digital TV Group, “Digital Terrestrial Television Requirements for Interoperability Issue 4.0” (Section 4.5 Audio description), Jan. 11, 2005).
However, the parameter may be altered into an inappropriate value, depending on the reception conditions. In such a case, the loudness cannot be appropriately adjusted, for example.